The invention relates to a scatterometer comprising a radiation source for providing a radiation beam and means for guiding the beam towards a sample location. The invention also relates to a method of operating a scatterometer.
Many surfaces of industrial products have a physical structure with certain properties as to enhance the functionality of the product or to improve its appearance. A few typical examples are the extremely smooth surfaces of high quality optical components, wear-resistant layers on cutting tools, the surface of paints, the finely textured plastic parts of surfaces of cosmetics packages and decorative personal care products, the pressing of rolling textures produced in sheet metal, and the high gloss metallic-looking laquers for the automotive industry.
These and many other products are said to have a surface texture. Texture is recognised as the property that determines the human interface, in other words generally how a product feels and looks. The xe2x80x9clooks-partxe2x80x9d of the texture is called the optical appearance. The optical appearance is a result of what the surface does with light inciding on the surface from the environment. Incident light comes from many directions in many cases and it can be reflected, transmitted, re-emitted, absorbed, coloured, diffused and scattered by surface roughness or structures or by the presence of small particles.
The assessment of textures for optical appearance is usually made in one or more of three methods: visual comparison, gloss and colour measurement, mechanical surface geometry measurement.
Visual assessments are made by visually comparing a product surface to certain standard textured surfaces by trained personnel. Visual appearance is governed by the geometrical outline of the surface and the optical properties of the material itself. Visual texture assessment of e.g. scratches is very difficult with light-coloured surfaces, because there the texture influence is overwhelmed by the strong reflection. White, moderately rough or fine surfaces do not look very different to the eye.
A gloss meter is a simple device that projects a light beam on the surface and measures the intensity ratio of the specular reflected beam and the diffused light in a halo around the specular reflection. This is done under fixed angles of incidence, often 30 or 60 degrees.
Mechanical microgeometrical measurements with contact probes (surface-tests) generate 1-D, 2-D or 3-D maps of the surface. By mathematical evaluation many statistical variants can be obtained by this method, like the well-known roughness measure Ra, average slope or peak counts. The method tries to find a relation between the optical appearance of the surface and its geometry.
The latter two methods assay to define certain figures of merit, derived statistically from observational data, that have a relation to the optical appearance.
In general terms, the limitations of these methods originate from the indirectness of the measured parameters. As an example, condensed statistical data derived from a surface measurement comprise a few merit figures that describe the geometrical shape of the surface texture quite well. The proposition is that there exists a relation between the geometrical data and how the surface looks to the eye. The subjective and personal factor makes this relation erratic and irreproducible.
As a second problem, there is the limitation of incomplete data fields. A gloss meter measures optical properties, but does so in a very limited way: only one incidence angle and two (direct and forward scattered) directions of reflection. This does no right to the wealth of variability of surfaces, angles of incidence effects, azimuthal effects, hemispherically strayed light. The optical appearance of a product is determined by the sum of all reflected light (or re-emitted by translucent materials) in the entire hemisphere, originating from incident light from the entire hemispherical environment.
The optical appearance of a textured surface (all surfaces have a texture, natural or purposely added) is formed by light reflected off this surface and entering the eye. Assuming a parallel light beam illuminating a moderately rough surface such as an automobile dashboard, part of the light reflects specularly in a certain direction, but a part also diffuses in other directions. Such a surface is said to be not perfectly diffuse. As seen in FIG. 1, the eye 1 captures light diffused off the product from different parts 3, 4 of the surface 2 under different angles. This light may have different intensities at different angles, so the surface may appear to have different brightness at different parts of the products, e.g. darker at lower angles.
Any light diffused in other directions than captured by the eye is lost to the eye altogether. The same is true for the usual form of gloss meters. If a light beam is aimed at a surface, it is necessary to measure the light diffused in all directions in the upper hemisphere, not integrated but with directional resolution, to fully describe the optical behaviour. To be complete, this should be done for incident light from any direction within the entire hemisphere, i.e. under all vertical (ascension) and horizontal (azimuth) angles. The full image of the directions and intensities of the diffused radiation can be obtained by measuring this hemispherical intensity distribution for multiple combinations of incident height and azimuth. The usual method for such measurements is to scan the entire hemisphere with a scatterometer with a moving detector as known from, inter alia, the German patent application no. 33 12 948. A complete measurement takes many hours in practice.
An object of the invention is to provide a scatterometer that can provide a reliable figure of merit of a surface in a relatively short time.
The object of the invention is achieved, when the scatterometer comprises a screen for intercepting radiation scattered from the sample location, and a radiation-sensitive detection system for capturing a two-dimensional image of the screen and converting it into an electric detector signal. The screen may be used in reflection or transmission and has the usual properties of a projection screen. The two-dimensional image formed on the screen represents the angular distribution of the radiation scattered by a sample arranged at the sample location. The image is therefore a Fourier-like transform of the physical properties of the sample, in which a spatial variation in physical properties of the sample is transformed to an angular variation of radiation energy. The use of an image detector, e.g. a video camera, allows a fast capture of the image, being the full distribution of the scattered radiation.
The screen may be flat, but it is preferably wholly or partly dome-shaped and centred substantially on the sample location, in order to capture all radiation scattered by the sample within an entire hemisphere. The dome itself may also be a hemisphere. The radiation source may be monochromatic, e.g. a semiconductor or other type of laser. For spectral investigations, such as reflection as a function of wavelength and angle of incidence, a polychromatic source, e.g. a white light source, may be used.
The sample at the sample location may be investigated in reflection or in transmission. In the latter case the incident beam and the scattered radiation to be detected are at opposite sides of the sample, and the measurement is indicative not only for the physical properties of the entrance and/or exit surface of the sample but also of its interior. The sample is preferably mounted on an adjustable stage, to allow changes in the azimuth of the sample. The image on the screen is preferably relayed to the detection system by a wide-angle optical system such as a fish-eye lens or a convex mirror which may-have the form of an off-axis mirror arranged close to the sample location.